Trichuris suis is a nematode parasite of swine which can cause serious pathology in infected pigs, such as anorexia, diarrhea, anemia, and death in heavy infections, especially in young animals. Infection with Trichuris suis is difficult to diagnose because much of the pathology occurs when only larval stages are present, and because of sporadic egg production by adult female worms. We have isolated a 20 kDa excretory/secretory (E/S) glycoprotein from culture fluids of adult worms by column chromatography and preparative gel electrophoresis which is diagnostic for Trichuris suis infection in pigs. The antigen did not crossreact in Western blots or enzyme-linked immunosorbent assay with sera from pigs infected with Ascaris suum, Trichinella spiralis, Oesophagostomum dentatum, or Toxoplasma gondii. The antigen was also diagnostic for Trichuris vulpis infection in dogs, and did not crossreact with sera from dogs infected with Toxocara canis, Ancylostoma caninum, or Strongyloides stercoralis. The antigen may be useful in clinical serologic assays for diagnosis of Trichuris infection in pigs and dogs in cases of scouring diarrhea of unknown etiology, and in epidemiological surveys of Trichuris suis infection in swine feeder/finishing operations.